1. Field of the Invention
The invention relates to a system and method for controlling a coasting downshift in a vehicle powertrain having an engine and an automatic transmission capable of producing multiple gear ratios.
2. Description of the Prior Art
Coasting or power-off downshifts are those gear ratio changes produced by the transmission automatically in response to a reduction in vehicle speed due to the need for a higher gear ratio. Power-off manual pull-in downshifts are those gear ratio changes initiated manually by the vehicle operator by moving a gear selector lever from a first position corresponding to the current gear to a second position corresponding to the next gear and by relieving force on the accelerator pedal. These actions allow the accelerator pedal to rise and the engine throttle to close, at least partially, in response to movement of the pedal. Such power-off downshifts, both those produced automatically by the transmission control system or those initiated manually by the operator and completed by the control system, are typically not as smooth as non-synchronous coasting downshifts.
Non-synchronous gearshifts are gear ratio changes resulting when a single friction element, a hydraulically actuated clutch or brake, changes its state from engaged to disengaged. Synchronous gear shifts are gear ratio changes resulting when at least two friction elements change state between engaged and disengaged.
During a synchronous downshift, the transmission gear ratio increases, which causes the speed of a torque converter turbine to rise in proportion to the ratio change. Because the engine is coupled to the transmission through the turbine, a power-off downshift causes the engine speed to increase as the turbine speed increases. The energy required to accelerate the engine inertia and friction torque load is supplied by vehicle inertia. This energy exchange causes the output torque supplied to the driven wheels to drop, resulting in a bumpy undesirable shift feel.
There is a need to improve the response of synchronous power-off downshifts and manual pull-in downshifts by controlling the increase in engine speed primarily through electronic throttle control and spark retardation, rather than by the on-coming friction element. Such a strategy would minimize the decrease in output shaft torque because energy required to raise engine speed and turbine speed would be provided by the engine.
A control strategy is required to improve the quality of coasting downshifts to a level equivalent to non-synchronous gear ratio changes. Such a strategy should be applicable both to synchronous and to non-synchronous downshifts.